The introduction of phased array antennas with the capability of beam steering and multiple beams have provided a major application of phase shifter and time delay networks used in the frequency microwave range. The ideal phase shifter network shifts the phase of the transmitted signal the same amount at all frequencies. The ideal time delay network delays the arrival of the signal at the output a specified amount of time. Both types of circuits can be used for beam steering. For pulse systems with short pulse lengths the transit time effects may demand the use of a more complex time delay circuit. Since no semiconductor component will alternate between a perfect short and a perfect open circuit the time delay or phase shifter circuit can be approximated only for over a limited frequency range.
The phase shifter or time delay circuit for microwave applications can be implemented with either a transmission or reflection type circuit. The transmission circuits can be realized as a (1) switched line circuit, (2) a low pass/high pass circuit, or (3) a loaded line circuit. The reflection circuit can be realized with either a circulator or a three dB coupler. At microwave frequencies the reflection circuit, which employs either a circulator or coupler, provides less insertion loss per degree of phase shift than any of the above-noted circuits. Such devices employ the minimum number of diodes or active devices per bit and any phase shift up to 180 degrees can be achieved. Also, the input match depends on the coupler or circulator rather than the diode switch or semiconductor device itself. However, the power handling capability of the coupler-type phase shifter is less than that obtained for the loaded line phase shifter.
A constant delay phase shifter utilizes an artificial transmission line constructed of high impedance transmission lines for the series inductive elements and field effect transistors (FETs) or diodes biased in the OFF state providing the shunt capacitance. This type of phase shifter operates by launching a signal down the artificial transmission line at the input end and allowing the signal to reflect back from a short at the output end. The circulator or coupler is required as indicated to separate the input signal from the output signal. Thus, the device operates to provide a phase shift proportional to twice that of the effective electrical length of the transmission line. The amount of the phase shift can be changed by changing the effective line length which is accomplished by a switching device, as for example, an FET or a PIN diode or other device, by turning it ON or OFF. Therefore an artificial transmission line fabricated using sixteen switching devices can provide sixteen 22.5.degree. phase shift steps at the center frequency of a particular band. If thirty two switching devices were used, 11.25.degree. phase shift steps at the band center frequency are obtained.
The reflection-type phase shifter, as described above, must isolate the input from the output. This is most commonly done with either the circulator or a 3 dB hybrid coupler. Such reflection-type phase shifters are known in the art and have been described in various publications. Many such reflection-type phase shifters use PIN diodes to operate as a switch to effectively short out sections of the transmission line. Either a PIN diode or a FET device can be employed at the junction or tap located between the inductances of the line. Practical FETs with large gate widths are required to insure low ON resistance which will therefore operate to produce an adequate short circuit. Large gate width FETs have relatively high OFF state capacitance which can make the construction of a reasonable impedance transmission line extremely difficult or if not impossible. As one can ascertain, most microwave systems use input and output impedances between 50 and 75 ohms for typical transmission lines and circuits.
The physical size of such FETs may prevent a practical implementation of an artificial transmission line utilized to accomplish the above-noted operation. A solution to this problem is to use smaller gate width FETs in parallel and turn more than one on which basically provides the same results as using a large FET. For example, if one biases or turns on four 150 micron gate width FETs rather than one 600 micron FET one achieves the same results as using the single larger FET. However, when small gate width FETs are operated in parallel then the effective resistance decreases, but the capacitance increases. This is not a good situation and creates further problems. Thus, to assure that the line is properly shorted at a selected tap all FETs not selected still have to be biased OFF or ON to assure reliable impedance values.
In order to obtain, for example, 16 or 32 phase shifts as described above, one needed a 16 or 32 bit word to implement a 4 or 5 bit control function. Therefore, the digital circuitry utilized to generate such words, as well as the decoders, was complex and involved additional cost.
According to the present invention there is described a circuit and control which eliminates complex digital circuitry by automatically switching all FETs at the transmission line taps to a fully conductive state between a selected FET at a given tap and the shorted end of the artificial transmission line. In this manner complex digital control circuits are eliminated, as well as efficient and reliable operation of the circuit configurations is provided.